HONG KONG—A Chinese property developer has agreed to bail out struggling rival Kaisa Group Holdings Ltd., in a move that could help soothe investor worries over China’s home builders.

Sunac China Holdings Ltd. Chairman Sun Hongbin told The Wall Street Journal in a telephone interview Wednesday that his company agreed to buy a 49.3% stake in Kaisa from its controlling family. He didn’t elaborate on the terms, but added that the agreement is pending approval from Hong Kong securities regulators.